A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
As the semiconductor industry moves into the deep submicron regime, the resolution limit of currently available lithographic techniques is being reached due to a decrease in the depth of focus, difficulty in the design of projection systems and complexities in the projection system fabrication technology. In order to address this issue, there have been continued endeavors to develop resolution enhancement techniques.
One of the resolution enhancement techniques that has been extensively used in small geometry semiconductor manufacturing to achieve process latitude and pattern resolution greater than that achievable using conventional binary masks is the phase shift mask. In today's rapidly advancing semiconductor manufacturing industry, which includes increasingly high levels of integration and correspondingly small feature sizes, the use of phase shift masks is important in the execution of state-of-the-art fabrication processes. So called attenuated phase shift masks (Att-PSM) are fabricated by replacing the opaque part of a conventional mask with a halftone film—one that is partially transmissive. The transmissivity of such a halftone film is generally on the order of about 10% transmission. The halftone film is chosen to desirably shift the phase of the radiation it transmits by 180 degrees. The radiation which passes through the clear area of the phase shift mask, in contrast, is not phase shifted. In this manner, destructive interference occurs between some diffracted waves which can be beneficial for imaging.
A main problem with Att-PSMs, however, is the potential printing of sidelobes, which are unwanted images in the final pattern caused by constructive interference between adjacent clear features in the mask pattern when the adjacent clear features are closely spaced and are separated by a distance on the order of the radiation wavelength. An example of sidelobe formation using a conventional lithographic apparatus is shown in FIGS. 1a-d. 
FIG. 1a is a schematic representation of a conventional lithographic apparatus 100 that includes a projection system 105 and a mask 110, which is used to selectively illuminate portions of a substrate 115 at least partially covered with a radiation sensitive material 120.
The mask 110 includes a radiation transmissive substrate 125, with a partially-transmissive phase-shifting material 130 on regions of the substrate 125. The phase-shifting material is a material which absorbs most of the radiation passing therethrough, and shifts the phase of the radiation which it does allow to pass therethrough. Radiation passing through the phase-shifting material in phase-shifting regions 130 is shifted in phase by approximately 180 degrees, thereby making it opposite in phase in comparison with radiation that passes through regions of the mask 110 which do not have a coating of phase-shifting material 130, such as the radiation transmissive or open region 140 shown in FIG. 1a. Radiation 145 passes through the mask 110 and exposes the radiation sensitive material 120 on the substrate 115.
The transmissivity of the mask 110 is plotted in FIG. 1b, wherein the transmissivity of the open region 140 is represented as a positive value 150 and the transmissivity of the regions with phase-shifting material 130 is represented as a negative value 155. The negative value for the transmissivity for regions of the mask 110 covered by the phase-shifting material 130 indicates the interference between radiation passing through open region 140 and radiation passing through phase-shifting regions.
The electric field intensity from the radiation 145 reaching the radiation sensitive material 120 is illustrated in FIG. 1c, the electric field intensity from the open region 140 being shown by a curve 160, and the electric field intensity from the phase-shifting regions being shown by a curve 165. The open region curve 160 is generally positive, with sinusoidal end portions 170 away from the open region 160, the end portions having asymptotically-reducing amplitude. The phase-shifting region curve 165 has a negative electric field intensity, the negative value being constant far from the open region 160, and reducing to zero in the vicinity of the transition between the phase-shifting regions and the open region 140.
The curves 160 and 165 are summed and squared to give the radiation exposure in the radiation sensitive material 120, as illustrated in FIG. 1d. The radiation exposure profile has a main peak 175 corresponding to the center of the open region 140. The radiation intensity drops off from the main peak to main troughs 180 on either side of the main peak 175. Moving further away from the main peak 175 are secondary peaks 185 and tertiary peaks 190. Far away from the main peak 175 is a constant exposure 195 which corresponds to the transmission of the attenuating material. A printing threshold intensity level, shown in FIG. 1d as a dashed line 196, is the minimum intensity level required for sufficient exposure of the radiation sensitive material 120 to eventually result in printing on the substrate 115. As illustrated, the intensity level results in printing a feature having a width 197 which is less than the width of the open region 140.
In order to avoid printing the secondary peaks 185 and tertiary peaks 190 on the substrate, the threshold 196 must be higher than the level of these peaks. However, even though this condition is satisfied, a secondary peak may combine with a secondary or tertiary peak from another feature (corresponding to another open region on the mask 110) to locally exceed the printing threshold. These undesired areas on the substrate where the printing radiation intensity exceeds the threshold intensity are referred to as “sidelobes”.
Sidelobes appear typically as spurious windows or ring structures in dense patterns, and are highly sensitive to pattern details (e.g. pitch) and optical conditions (e.g. source shape and numerical aperture NA). Sidelobe printing is most problematic for hole patterns with pitches near 1.2*λ/NA (where λ is the radiation wavelength and NA is the numerical aperture of lithographic apparatus), with small pattern bias (i.e. relatively high printing dose) and where high transmission masks are used. Sidelobe printing also may be problematic for 193 nm lithography, where current radiation sensitive materials may not have sufficient surface inhibition to prevent sidelobe formation. Sidelobe printing may drastically affect device manufacturing yield because the unwanted additional features in the pattern may be transferred into the substrate.